This invention generally relates to two-pack coating compositions used in automotive coating applications, particularly in automotive refinish applications. More particularly, this invention relates to a coating composition that cures in two stages under ambient or low bake conditions.
Solvent-based coating compositions generally include a binder polymer and a crosslinking agent, which cross link upon application to produce coatings having excellent coating properties. One problem associated with such coating compositions is the relatively longer time required to cure these compositions. Such longer cure times cut down the productivity in automotive refinish shops by requiring the automobile or truck to remain for a longer period of time in the area in which it was spray coated. A rapid initial cure is thus desirable to produce a coating that can be readily sanded or buffed without fouling sandpaper. Such a rapid initial cure permits the user to readily remove coated automobile or trucks bodies out of the spray booths and allow them to fully cure at other convenient locations. As a result, productivity of coating autobodies can be improved substantially.
Attempts have been made to decrease the curing time of these two pack-coating compositions by using more reactive components or catalysts. However, while the use of such components decreases curing time, the higher reactivity of such components usually reduces the time to gelation or pot life of the coating composition as well. In the most extreme case, instant gelation of the composition can occur when, for example, the polyisocyanate is mixed with a polymer having reactive amine groups. Thus, a need still exists for a two-pack coating composition that cures rapidly while still having an acceptable pot life.
Attempts have been made to provide for a coating composition that cures rapidly after application. A commonly assigned U.S. Pat. No. 5,859,136 discloses a coating composition containing a dispersed core-shell acrylic polymer having a core polymerized from ethylenically unsaturated monomers containing amine functional groups. These amine groups positioned in the core are not readily available to react with the crosslinking agent, such as an isocyanate, which has to penetrate the shell to reach the core to crosslink with amine functionality. Applicants have come up with a novel alternate approach to reduce the cure time of the coating composition while still providing acceptable pot life and coating properties.
The present invention is directed to a two-stage cure coating composition comprising a binder and a crosslinking component,
said binder component comprising an acrylic polymer having in the range of from 5.0 weight percent to 70.0 weight percent of hydroxyl moieties and in the range of from 0.5 weight percent to 8.0 weight percent of secondary amine moieties, all percentages based on the weight of binder component solids; and
said crosslinking component comprising a crosslinking agent having at least two isocyanate groups.
The present invention is also directed to a method of producing a coating on a substrate, said method comprising:
mixing a binder component and a crosslinking component of a two-stage cure coating composition to form a pot mix, wherein said binder component comprises an acrylic polymer having in the range of from 5.0 weight percent to 70.0 weight percent of hydroxyl moieties and in the range of from 0.5 weight percent to 8.0 weight percent of secondary amine moieties, all percentages based on the weight of binder component solids; and wherein said crosslinking component comprises a crosslinking agent having at least two isocyanate groups;
applying a layer of said pot mix over said substrate;
first stage curing said layer, wherein said layer has a Persoz hardness of at least 30 within 2 hours after said application;
second stage curing said first stage cured layer into said coating on said substrate.
One of the advantages of the coating composition of the present invention is that a coating resulting therefrom has rapid first stage cured state (defined below) obtained under ambient conditions when compared against the coatings obtained from conventional coating compositions.
Another advantage of the present invention is that a coating resulting therefrom still provides excellent second stage cured state (defined below) necessary to achieve desired long term coating properties, such as etch and mar resistance.
Still another advantage of the present invention is that it requires substantially low amounts of solvent needed for efficient spray application, thus enabling the formulator, especially in the United States, to meet the increasingly stringent legal requirements that deal with the acceptable level of release of VOC (volatile organic content) in the atmosphere.
Still another advantage of the coating composition of the present invention is its extended pot life as compared to conventional rapid curing coating compositions.
The novel coating composition of the present invention also advantageously permits a formulator to select from a wider selection of other film forming polymers that can be incorporated in the coating composition.
As defined herein:
xe2x80x9cTwo-pack coating compositionxe2x80x9d means a thermoset coating composition comprising two components stored in separate containers. These containers are typically sealed to increase the shelf life of the components of the coating composition. The components are mixed prior to use to form a pot mix. The pot mix has a limited pot life typically of minutes (about 30 minutes to 60 minutes) to a few hours (1 hour to 2 hours). The pot mix is applied as a layer, typically through a spray nozzle, of desired thickness on a substrate surface, such as an autobody. After application, the layer is cured under ambient conditions or bake cured at elevated temperatures to form a coating on the substrate surface having desired coating properties, such as high gloss, mar-resistance and resistance to environmental etching.
xe2x80x9cGPC weight average molecular weightxe2x80x9d and xe2x80x9cGPC number average molecular weightxe2x80x9d means a weight average molecular weight (Mw) and a number average molecular weight (Mn), respectively measured by utilizing gel permeation chromatography. A high performance liquid chromatograph (HPLC) supplied by Hewlett-Packard; Palo Alto, Calif. was used. Unless stated otherwise, tetrahydrofuran was used as the liquid phase and polystyrene was used as the standard.
xe2x80x9cPolydispersityxe2x80x9d of a polymer is a ratio of Mw to Mn.
xe2x80x9c(Meth)acrylatexe2x80x9d means methacrylate and acrylate.
xe2x80x9cPolymer solidsxe2x80x9d or xe2x80x9ccomposition solidsxe2x80x9d means a polymer or composition in its dry state.
xe2x80x9cFirst stage curexe2x80x9d under ambient conditions occurs when a layer of a coating composition upon application cures sufficiently within a very short duration of less than two hours to produce a surface that can be readily buffed or sanded without fouling the sanding paper. Such a first stage cured layer generally has a Persoz hardness of about 30 or more, preferably about 35 or more and most preferably in the range of about 40 to about 100. The procedure used for determining the Persoz hardness is described in the Examples section below.
xe2x80x9cSecond stage curexe2x80x9d of the first stage cured layer under ambient conditions occurs within several days (generally about 2 days to about 10 days) to produce a fully cured coating that is tough and hard and has the desired coating properties, such as etch and mar resistance. If desired, the second cure time can be shortened to about 12 hours to 48 hours by bake curing it for about 60 to 15 minutes at about 60xc2x0 C. to 80xc2x0 C.
The novel composition of this invention has an excellent pot life of about 1 to 2 hours. A layer from the novel composition of this invention dries rapidly to the first stage cure at ambient temperatures. Thereafter, within a few days, the first stage cured layer fully cures (the second stage cure) at ambient temperatures to produce a hard tough coating. This is particularly advantageous in refinishing automobiles and trucks. For example, in repairing a clear coat/color coat finish of an automobile or truck, generally the color coat is applied and dried for a short time but not cured and then the clear coat is applied and both coats are cured, all at ambient temperatures. If necessary, the cured clear coat is sanded and buffed to improve appearance and remove minor imperfections. For a clear finish to be sandable and buffable, it must be hard but not tough. Since, the coating composition of this invention has a short first. stage cure time; the rate of processing vehicles through a typical repair facility can be substantially increased. Thus, the vehicle can be moved out of the spray booth area to provide room for another vehicle to be painted. Similarly, if the present composition is used as a primer, it can be sanded in a short period of time after application and a topcoat can then be applied on top of the sanded surface.
These advantages of the novel composition are the result of having a reactive functional group in the acrylic polymer utilized in the composition. However, since the secondary amine group is highly reactive, a coating composition containing such reactive groups has unacceptably short pot life. Applicants have discovered that by utilizing a certain amount of such short acting reactive groups, for example, amine moieties in the polymer, it becomes possible to produce a coating composition that has an acceptable pot life and still produce coatings having an acceptable first stage cure time. However, such a coating composition in and of itself would not produce coatings having desirable coating properties. Applicants elegantly addressed that problem by providing long acting reactive groups, such as hydroxyl moieties, in a polymer of certain molecular weight and glass transition temperature. Thus, after the dual cure coating composition of the present invention is applied over a substrate and after solvent evaporates during the drying process, the short acting moieties in the polymer become available to rapidly react with the crosslinking agent and form a crosslinked finish that is tack free in a short period. The long acting reactive groups in the polymer thereafter react with the crosslinking agent to attain second stage cure in a relatively short time at ambient temperatures to produce a coating having hard durable finish and excellent coating properties.
The coating composition of the present invention is a two-pack coating composition that includes a binder component and a crosslinking component. These components are stored separately, for example, in separate containers and are mixed just prior to use to form a pot mix. The coating composition generally includes in the range of 50 weight percent to 90 weight percent of the binder component and includes in the range of 10 weight percent to 50 weight percent of the crosslinking component, all percentages being based on the composition solids.
The binder component includes in the range of 30 weight percent to 90 weight percent, preferably in the range of 40 weight percent to 80 weight percent, and more preferably in the range of 50 weight percent to 70 weight percent of an acrylic polymer, all percentages being based on the binder component solids.
Applicants have unexpectedly discovered that by providing an acrylic polymer with a certain number of pendant secondary amine moieties, rapid first stage cure can be attained. However, since the reactivity of the secondary amine with a typical crosslinking agent, such as polyisocyanate, is extremely fast, applicants have unexpectedly discovered that by adding just a small amount of a secondary amine monomer during acrylic polymer polymerization, the first stage cure time can be dramatically reduced from 4 to 8 hours to about 2 hours without any significant attenuation in the pot life.
The foregoing result was quite unexpected and it was attained by including in the binder component an acrylic polymer having in the range of from 0.5 weight percent to 8.0 weight percent, preferably in the range of 0.75 weight percent to 6.0 weight percent, and more preferably in the range of 1.0 weight percent to 5.0 weight percent of secondary amine moieties, all percentages based on the weight of binder component solids. If the amount of the secondary amine moieties used exceeds the foregoing upper limit, the pot mix becomes too viscous to be of any practical spray application, if the amount of the secondary amine moieties used is less than the foregoing lower limit, no significant improvement in the first stage cure takes place.
The secondary moieties can be derived from polymerizing the acrylic polymer monomer from one or more suitable secondary amine monomers. For example tert-butyl aminoethyl (meth)acrylate. Tert-butyl aminoethyl methacrylate is preferred. Alternatively, the secondary amine functionality may be introduced by post reacting a polymer containing glycidyl (meth)acrylate with a primary amine or an alkanol amine having primary amine groups. Suitable primary amines include propyl amine, butyl amine, hexyl amine, octyl amine and benzyl amine. Suitable alkanol amines include ethanol amine, propanol amine, butanol amine and methyl ethanol amine.
Applicants also discovered that though the presence of the secondary amine moieties substantially reduces the first stage cure time, some other means are needed to attain a desired degree of second stage cure, which is essential for a high quality coating. Applicants unexpectedly discovered that by providing the acrylic polymer with dual functionalities, desired second stage cure could be attained without sacrificing the fast first stage cure or without any significant decrease in the pot life. Applicants attained the foregoing results by providing the acrylic polymer with a certain number of pendant hydroxyl moieties in addition to the aforedescribed secondary amine moieties. Thus, the acrylic polymer further includes in the range of 5.0 weight percent to 70.0 weight percent, preferably in the range of 10.0 weight percent to 40.0 weight percent, and more preferably in the range of 15.0 weight percent to 30.0 weight percent of hydroxyl moieties, all percentages based on the weight of the binder component solids. If the amount of the hydroxyl moieties used exceeds the foregoing upper limit, the resultant coating will tend to crack, if the amount of the hydroxyl moieties used is less than the foregoing lower limit, the resultant coating will have a tacky feel to it.
Preferred hydroxy moieties are derived from hydroxy monomers, such as hydroxy alkyl (meth)acrylates wherein the alkyl group has in the range of 1 to 4 carbon atoms in the alkyl group. Exemplars include hydroxy ethyl (meth)acrylate, hydroxy propyl (meth)acrylate, hydroxy butyl (metha)crylate or a combination thereof. Hydroxy ethyl methacrylate is preferred
The monomer mixture will include other suitable monomers, such as, styrene, alkyl styrene; vinyl toluene; acrylonitrile; alkyl (meth)acrylates having 1-18 carbon atoms in the alkyl group, such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, butyl (meth)acrylate, pentyl (meth)acrylate, hexyl (meth)acrylate, 2-ethyl hexyl (meth)acrylate, nonyl (meth)acrylate, lauryl (meth)acrylate, stearyl (meth)acrylate; cycloaliphatic (meth)acrylates, such as trimethylcyclohexyl (meth)acrylate, and isobutylcyclohexyl (meth)acrylate; aryl (meth)crylates, such as benzyl (meth)acrylate; isobornyl (meth)acrylate; cyclohexyl (meth)acrylate; glycidyl (meth)acrylate, ethyl hexyl (meth)acrylate, benzyl (meth)acrylate or a combination thereof. Methcraylates of methyl, butyl, n-butyl, and isobornyl are preferred.
The acrylic polymer suitable for use may be a linear polymer, a branched polymer, a core-shell polymer, or a combination thereof. The linear acrylic polymer is preferred. Applicants also unexpectedly discovered that to attain the aforedescribed application productivity advantages, the acrylic polymer preferably has a molecular weight and a Tg in a certain range. Thus, the acrylic polymer which may be a linear or branched acrylic polymer has a weight average molecular weight (Mw) varying in the range of from 1,000 to 30,000, preferably varying in the range of from 1,500 to 20,000, more preferably varying in the range of from 2000 to 15,000 and a Tg varying in the range of from of xe2x88x9220xc2x0 C. to 100xc2x0 C., preferably varying in the range of from 0xc2x0 C. to 90xc2x0 C., and more preferably varying in the range of from 20xc2x0 C. to 80xc2x0 C.
The linear acrylic polymer may be produced by conventional processes well known in the art. Typically, solvent is added to a reactor and brought to reflex at elevated temperatures under a nitrogen blanket. Optionally, before adding heat, the reactor may be fed with a portion of the monomer mixture and one or more typical initiator, such as the azo type catalysts, which include 2,2xe2x80x2-azobis (2,4 dimethylpentane nitrile); peroxides, such as di-tertiarybutyl peroxide; and hydroperoxides. Commercially available peroxy type initiator t-butylperoxide or Triganox(copyright) B from AKZO NOBEL is suitable for use in the present invention. Upon attaining the desired polymerization temperature, the initiator and the monomer mixture are simultaneously fed to the reactor over a period of time. Optionally, a shot of secondary amine monomer may be added towards the end of polymerization. Sometimes, it is also desirable to add additional initiator upon completion of addition of the monomer mixture to ensure completion of the polymerization process.
The branched acrylic polymer can be produced by a polymerization process, described in U.S. Pat. Nos. 4,680,352 and 5,290,633, which are incorporated herein by reference. Typically, the branched polymers are made in two stages. In the first stage, macromonomers, using conventional cobalt (II) or (III) chelate chain transfer agent, are produced to ensure that the macromonomer is provided with one terminal ethylenically unsaturated group, which is polymerizable. During the second stage, the monomer mixture described earlier is added to the reactor containing the macromonomers. The monomers polymerize with the ethylenically unsaturated group on the macromonomer to produce the branched acrylic polymer.
The core-shell polymer has a solvent insoluble core, and a solvent soluble shell, chemically attached to the core. Preferably, the shell is in the form of macromonomer chains or arms attached to it. The core-shell polymer is a polymer particle dispersed in an organic media, wherein the polymer particle is stabilized by what is known as steric stabilization. The average particle size of the core ranges from 0.1 to 1.0 microns, preferably from 0.15 to 0.6, more preferably from 0.15 to 0.6.
The core-shell polymer includes in the range of from about 10 percent to 90 percent, preferably in the range of from 50 percent to 80 percent all in weight percent based on the weight of the dispersed polymer, of a core formed from high molecular weight polymer having a weight average molecular weight of about 50,000 to 500,000, preferably in the range of from 50,000 to 200,000, more preferably in the range of from 50,000 to 150,000. The arms make up about 10 percent to 90 percent, preferably 20 percent to 50 percent, all in weight percent based on the weight of the core-shell polymer. The arms are formed from a low molecular weight polymer having weight average molecular weight in the range of from about 1,000 to 50,000, preferably in the range of from 2000 to 40,000, more preferably in the range of from 3000 to 30,000.
The core of the dispersed core-shell polymer is comprised of one or more polymerized acrylic monomers. Suitable monomers include styrene, alkyl (meth)acrylate having alkyl carbon atoms in the range of from 1 to 18, preferably in the range of from 1 to 12; ethylenically unsaturated monocarboxylic acid, such as, (meth)acrylic acid, silane-containing monomers, and epoxy containing monomers, such as glycidyl (meth)acrylate. Other optional monomers include amine containing monomers, hydroxyalkyl (meth)acrylate or acrylonitrile. Optionally, the core may be crosslinked through the use of diacrylates or dimethacrylates, such as, allyl methacrylate or through post reaction of hydroxyl moieties with polyfunctional isocyanates or carboxylic moieties with epoxy moieties.
The macromonomer arms attached to the core are polymerized from the hydroxyl and secondary amine monomers, described earlier. In addition, the arms may be polymerized from monomers, such as styrene and alkyl (meth)acrylates having 1 to 12 carbon atoms.
The process for making the core-shell polymer is described in U.S. Pat. No. 5,659,136, which is incorporated herein by reference.
The crosslinking component of the coating composition of the present invention includes one or more crosslinking agents having at least two isocyanate groups, such as a polyisocyanate crosslinking agent. Any of the conventional aromatic, aliphatic, cycloaliphatic, isocyanates, trifunctional isocyanates and isocyanate functional adducts of a polyol and a diisocyanate can be used. Typically useful diisocyanates are 1,6-hexamethylene diisocyanate, isophorone diisocyanate, 4,4xe2x80x2-biphenylene diisocyanate, toluene diisocyanate, bis cyclohexyl diisocyanate, tetramethylene xylene diisocyanate, ethyl ethylene diisocyanate, 2,3-dimethyl ethylene diisocyanate, 1-methyltrimethylene diisocyanate, 1,3-cyclopentylene diisocyanate, 1,4-cyclohexylene diisocyanate, 1,3-phenylene diisocyanate, 1,5-naphthalene diisocyanate, bis-(4-isocyanatocyclohexyl)-methane and 4,4xe2x80x2-diisocyanatodiphenyl ether.
Typical trifunctional isocyanates include triphenylmethane triisocyanate, 1,3,5-benzene triisocyanate and 2,4,6-toluene triisocyanate. Trimers of diisocyanates also can be used, such as the trimer of hexamethylene diisocyanate, which is supplied by Bayer Corporation, Pittsburgh, Pa., under the trademark Desmodur(copyright) N-3390. Other suitable polyisocyanates from Bayer Corporation include Desmodur(copyright) N-3300, and Z-4470BA polyisocyanates.
The relative amount of crosslinking agent used in the coating composition is adjusted to provide a molar equivalent ratio of NCO/(OH+NH) in the range of from 0.5 to 2, preferably in the range of from 0.75 to 1.5 and more preferably in the range of from 0.85 to 1.25.
The coating composition preferably includes one or more catalysts to enhance crosslinking of the components during curing. Generally, the coating composition includes in the range of from 0.005 percent to 2 percent, preferably in the range of from 0.01 to 1 percent and more preferably in the range of from 0.02 percent to 0.7 percent of the catalyst, the percentages being in weight percentages based on the total weight of the binder and crosslinking component solids. These catalysts are preferably added to the binder component.
Applicants also discovered that the pot life of the coating composition containing the aforedescribed catalyst can be extended by adding in the range of from 0.1 weight percent to 2.0 weight percent, preferably in the range of from 0.15 weight percent to 1.0 weight percent and more preferably in the range of from 0.2 weight percent to 0.5 weight percent a pot life extending agent.
One of the suitable pot life-extending agents is carboxylic acid, such as acetic acid, propionic acid, butyric acid, lauric acid. Acetic acid is preferred.
If desired, the binder component of the coating composition may also include one or more oligomers having a weight average molecular weight (Mw) in the range of from 100 to 2000, preferably in the range from 500 to 1500, a polydispersity in the range of from 1.01 to 1.7, preferably in the range of from 1.05 to 1.5 and more preferably in the range from 1.1 to 1.3, and having one or more isocyanate reactive functionalities. The oligomer preferably includes in the range from 2 to 12, more preferably in the range from 2 to 8 and most preferably in the range from 2 to 6 isocyanate reactive functionalities. The suitable isocyanate reactive functionalities include a hydroxyl group, epoxy group or a combination thereof.
The oligomer can be produced by first reacting a multifunctional alcohol, such as, pentaerythritol, hexandiol, trimethyol propane with alicyclic monomeric anhydrides, for example, hexahydrophthalic anhydride or methylhexahydrophthalic anhydride to produce an oligomeric acid. Oligomeric acids having at least one hydroxyl functionality are also suitable, prepared by reacting the multifunctional alcohol with less than a stochiometric amount of the monomeric anhydride.
The oligomeric acid is then reacted with a monofunctional epoxy under pressure at a reaction temperature in the range of from 60xc2x0 C. to 200xc2x0 C. Typical reaction time is in the range of from 1 hours to 24 hours, preferably 1 hour to 4 hours. The foregoing two-step process ensures that the hydroxyl functionalities are uniformly distributed on each oligomeric chain of the reactive oligomer to produce the reactive oligomers with the polydispersity in the range described earlier. The monofunctional epoxy suitable for use in the present invention include alkylene oxide of 2 to 12 carbon atoms, ethylene, propylene and butylene oxides are preferred, ethylene oxide is more preferred. Other epoxies, such as, Cardura(copyright) E-10 glycidyl ester, supplied by Exxon Chemicals, Houston, Tex. may be used in conjunction with the monofunctional epoxies, described above. The details of producing the oligomer are described in a PCT Publication WO99/23131, which was published on May 14, 1999. Said publication is incorporated herein by reference.
If desired, the coating composition may include an acrylic resin, polyester or a combination thereof.
The polyester has at least one or more of the aforedescribed isocyanate reactive functionalities, a weight average molecular weight (Mw) varying in the range of from 2000 to 20,000, preferably varying in the range of from 3000 to 10,000 and a Tg varying in the range of from of xe2x88x9220xc2x0 C. to 100xc2x0 C., preferably varying in the range of from 0xc2x0 C. to 90xc2x0 C., and more preferably varying in the range of from 20xc2x0 C. to 80xc2x0 C.
The polyester suitable for use in the present invention may be any conventional polyester conventionally polymerized from suitable polyacids, including cycloaliphatic polycarboxylic acids, and suitable polyols, which include polyhydric alcohols. Examples of suitable cycloaliphatic polycarboxylic acids are tetrahydrophthalic acid, hexahydrophthalic acid, 1,2-cyclohexanedicarboxylic acid, 1,3-cyclohexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, 4-methylhexahydrophthalic acid, endomethylenetetrahydrophthalic acid, tricyclodecanedicarboxylic acid, endoethylenehexahydrophthalic acid, camphoric acid, cyclohexanetetracarboxylic acid and cyclobutanetetracarboxylic acid. The cycloaliphatic polycarboxylic acids can be used not only in their cis but also in their trans form or a mixture thereof. Examples of suitable polycarboxylic acids, which, if desired, can be used together with the cycloaliphatic polycarboxylic acids, are aromatic and aliphatic polycarboxylic acids, such as, for example, phthalic acid, isophthalic acid, terephthalic acid, halogenophthalic acids, such as, tetrachloro- or tetrabromophthalic acid, adipic acid, glutaric acid, azelaic acid, sebacic acid, fumaric acid, maleic acid, trimellitic acid, and pyromellitic acid.
Suitable polyhydric alcohols include ethylene glycol, propanediols, butanediols, hexanediols, neopentylglycol, diethylene glycol, cyclohexanediol, cyclohexanedimethanol, trimethylpentanediol, ethylbutylpropanediol, ditrimethylolpropane, trimethylolethane, trimethylolpropane, glycerol, pentaerythritol, dipentaerythritol, tris(hydroxyethyl) isocyanate, polyethylene glycol and polypropylene glycol. If desired, monohydric alcohols, such as, for example, butanol, octanol, lauryl alcohol, ethoxylated or propoxylated phenols may also be included along with polyhydric alcohols. The details of polyester suitable for use in the present invention are further provided in the U.S. Pat. No. 5,326,820, which is incorporated herein by reference. One example of the commercially available polyester suitable for use is SCD(copyright)-1040 polyester, which is supplied by Etna Product Inc., Chagrin Falls, Ohio.
The acrylic resin has at least one or more of the aforedescribed isocyanate reactive functionalities, a weight average molecular weight (Mw) varying in the range of from 1,000 to 30,000, preferably varying in the range of from 1,500 to 20,000, more preferably varying in the range of from 2000 to 15,000 and a Tg varying in the range of from of xe2x88x9220xc2x0 C. to 100xc2x0 C., preferably varying in the range of from 0xc2x0 C. to 90xc2x0 C., and more preferably varying in the range of from 20xc2x0 C. to 80xc2x0 C.
The aforedescribed acrylic resin may be conventionally prepared in accordance with the process disclosed in the U.S. Pat. No. 5,286,782, which is incorporated herein by reference.
Some of the suitable solvents include aromatic hydrocarbons, such as petroleum naphtha or xylenes; esters, such as, butyl acetate, t-butyl acetate, isobutyl acetate or hexyl acetate; and glycol ether esters, such as propylene glycol monomethyl ether acetate. The amount of organic solvent added depends upon the desired solids level as well as the desired amount of VOC of the composition. If desired, the organic solvent may be added to both the components of the coating composition.
The amount of solvent added to the coating composition may be adjusted to provide the composition with a VOC (volatile organic content) in the range of from 0.12 kilograms (1.0 pounds per gallon) to 0.78 kilograms (6.5 pounds per gallon) of the solvent per liter of the coating composition.
The coating composition of the present invention may also contain conventional additives, such as stabilizers, and rheology control agents, flow agents, and toughening agents. Such additional additives will, of course, depend on the intended use of the coating composition. Any additives that would adversely effect the clarity of the cured coating will not be included when the composition is used as a clear coating. The foregoing additives may be added to either component or both, depending upon the intended use of the coating composition.
To improve weatherability of the coating, 0.1 to 5 weight percent, preferably 0.5 to 2.5 weight percent and more preferably 1 to 2 weight percent of ultraviolet light stabilizers screeners, quenchers and antioxidants can be added to the composition, the percentages being based on the total weight of the binder and crosslinking components solids. Typical ultraviolet light screeners and stabilizers include the following:
Benzophenones, such as hydroxy dodecycloxy benzophenone, 2,4-dihydroxy benzophenone, and hydroxy benzophenones containing sulfonic acid groups.
Benzoates, such as dibenzoate of diphenylol propane and tertiary butyl benzoate of diphenylol propane.
Triazines, such as 3,5-dialkyl-4-hydroxyphenyl derivatives of triazine and sulfur containing derivatives of dialkyl-4-hydroxy phenyl triazine, hydroxy phenyl-1,3,5-triazine
Triazoles, such as 2-phenyl-4-(2,2xe2x80x2-dihydroxy benzoyl)-triazole and substituted benzotriazoles, such as hydroxy-phenyltriazole.
Hindered amines, such as bis(1,2,2,6,6entamethyl-4-piperidinyl sebacate) and di[4(2,2,6,6,tetramethyl piperidinyl)]sebacate; and any mixtures of any of the above.
In use, the first-pack of the two-pack coating composition containing the binder component and the second-pack containing the crosslinking component are mixed just prior to use or about 5 to 30 minutes before use to form a pot mix. A layer of the pot mix is typically applied to a substrate by conventional techniques, such as spraying, electrostatic spraying, roller coating, dipping or brushing. Generally, a layer having a thickness in the range of from 25 micrometers to 75 micrometers is applied over a metal substrate, such as automotive body, which is often pre-coated with other coating layers, such as an electrocoat, primer and a basecoat.
In applying the clear coating composition to a vehicle such as an automobile or a truck for a repair or repainting, the basecoat which may be either a solvent based composition or a waterborne composition is first applied and then dried to at least remove solvent or water before the clear coat is applied usually by conventional spraying. Electrostatic spraying also may be used. The clear coat is dried at ambient temperatures but moderately higher temperatures of up to about 80xc2x0 C. can be used. As soon as the clear finish is dust free and tack free the vehicle can be moved from the work area to allow for the refinishing of another vehicle.
Generally, within about 2 hours after application, the layer from the pot mix of the coating composition cures to the first stage, i.e., it is sufficiently cured to allow for buffing and polishing, if needed, to remove imperfections and improve gloss of the finish. The first stage cured layer continues to cure and after several days, it reaches a level of hardness and toughness required for a durable and weatherable automotive finish, i.e., the coating reaches the second stage cured state.
The coating composition of the present invention is suitable for use as a clear or pigmented composition. The composition can be pigmented with conventional pigments, including metallic flakes. The coating composition can be used as a monocoat or as a basecoat or as a primer.
The coating composition of the present invention is suitable for providing coatings on a variety of substrates, such as metal, wood and concrete substrates and resinous surfaces, such as, for example, RIM (reaction injection molded) auto bumpers and dashboards. The present composition is suitable for providing clear or pigmented coatings in automotive OEM (original equipment manufacturer) applications and especially suitable for refinish applications typically used in making repairs and touch-ups to automotive bodies. Obviously, the coating composition is also well suited for use in other applications, such as coating truck bodies, boats, airplanes, tractors, cranes and other metal bodies. The coating composition of the present invention is also suitable for use in industrial and maintenance coating applications.